Salvadoran President Jose Napoleon Duarte said yesterday that he will seek to "humanize the war" in his country by insisting that leftist rebels talk privately with him about ending civilian casualties before he agrees to any more public negotiating sessions.

Duarte told President Reagan in a half-hour meeting that the Salvadoran armed forces are under "tight rules of engagement" to minimize civilian suffering, a senior administration official told reporters.

Duarte "is deeply determined to see what he can do to get the guerrillas to cease attacking the economy . . . to cease using assassination and terror against the civilian populations as weapons in the war," the official said.

U.S. critics of the Duarte government have charged that army planes have bombed the civilian population indiscriminately, causing dozens of deaths, to discourage support for the guerrillas who are seeking Duarte's overthrow. Duarte has denied the charges.

Duarte and the rebels met twice last year to try to get peace talks going, but little progress was made. The rebels suggested last week that a third meeting be held June 10 in El Salvador but Duarte countered with a proposal for private discussions outside the country.

Guillermo Ungo, a leader of the Salvadoran guerrillas' political wing, said by telephone from Mexico City that Duarte had not outlined a specific agenda and that Duarte's announcement yesterday was an effort to avoid a third public negotiating session.

"We don't want to have private talks as a substitute for talks that are agreed upon," Ungo said.

After meeting with Secretary of State George P. Shultz, Duarte told reporters that his "democratic revolution" requires a new approach to talks.

"I can't risk the people's faith by going to any meeting just to make a show," Duarte said.

The senior administration official called Duarte's visit "the opening of a new chapter in our relationship" after five years of joint antiguerrilla warfare. There have been "some spectacular successes" in combat and in reducing human rights violations, the official said, so that "it's possible now . . . to give much more serious attention to some of the other problems" of El Salvador's devastated economy.

"There's absolutely no doubt that there's been a monumental change in the nature of the situation there," the official said.

He added that the administration would be "sympathetic" to Duarte's expected requests for new economic and military aid, but that no decision has been made to request more funds in fiscal 1985.

Asked whether the United States would supply more armed C47 cargo planes, the official said that "we would be sympathetic, if the need is there."

Duarte has already received two armed C47s and is in the process of receiving five that are "gunship-capable" but currently are unarmed, according to another official.

Reagan chided Nicaragua for failing to follow Duarte's example in talking with its domestic insurgents, and rebuked Congress for failing to give as much support to administration policies in Nicaragua as it has to his policy in El Salvador.

"Those who question our efforts in Central America should take note of the heartwarming progress that President Duarte has made," Reagan said in the White House Rose Garden. "We must have the courage to help all our friends in Central America."

Reagan also thanked Duarte for supporting his efforts to win funds for antigovernment rebels in Nicaragua and for backing the U.S. trade embargo against Nicaragua. Only El Salvador and Honduras have expressed support for the U.S. economic sanctions.

Earlier, national security affaris adviser Robert C. McFarlane met with House Minority Leader Robert H. Michel (R-Ill.) and other GOP members to decide on strategy for a renewed attempt to fund the Nicaraguan rebels. The staffs of several Democratic senators also met to consider counterproposals, and debate is expected to resume next week on both sides of Capitol Hill.

Duarte, who is beginning a nine-day visit to the United States, met with Defense Secretary Caspar Weinberger and Vice President Bush. Duarte is scheduled to receive an honorary doctorate on Sunday from his alma mater, the University of Notre Dame, and to return to Washington next week to lobby Congress for additional military and economic assistance.